How to Add Custom Policy Routing for Multiple NICs on Linux

By default Linux routes all traffic through the primary interface (eth0). When a server has a second NIC (eth1) with its own IP address and gateway, you must add extra routes so that traffic to and from that subnet uses eth1.

In the example, eth0 is configured with IP 19.86.101.54/24 and gateway 19.86.101.1, while eth1 uses IP 19.86.100.176/24 and gateway 19.86.100.1. The ifconfig -a output shows both interfaces, and netstat -rn confirms the default route points to eth0.

To direct traffic through eth1, create a new policy routing table. First back up the existing table file, then add an entry named admin:

cd /etc/iproute2
cp rt_tables rt_tables.orig
echo "1 admin" >> /etc/iproute2/rt_tables

Next, add routes to the admin table for the eth1 subnet and its default gateway:

ip route add 19.86.100.0/24 dev eth1 src 19.86.100.176 table admin
ip route add default via 19.86.100.1 dev eth1 table admin

Then create rules so that any traffic originating from or destined to the eth1 IP range uses the admin table:

ip rule add from 19.86.100.176/24 table admin
ip rule add to 19.86.100.176/24 table admin
ip route flush cache

After applying these commands, ip rule show should list the new entries (e.g., 32764 and 32765) above the default main lookup, ensuring the rules are evaluated first.

Verify the configuration by pinging the eth1 address from an external network; the traffic should now flow through eth1. To make the changes persistent across reboots, add the commands to /etc/init.d/boot.local on SUSE or /etc/rc.d/rc.local on Red Hat/CentOS. Repeat the steps with a different table name if you need additional sub‑net configurations.